How to unmap a file from memory mapped using FileChannel in java?

Following static method could be used:

public static void unmap(MappedByteBuffer buffer)
{
   sun.misc.Cleaner cleaner = ((DirectBuffer) buffer).cleaner();
   cleaner.clean();
}

But this is unsafe solution because of following:
1) Lead to failures if someone use MappedByteBuffer after unmap
2) It relies on MappedByteBuffer implementation details